1. Field of the Invention
The present invention relates to a process for the correction of the path of a trajectory, effected in accordance with an expected target offset, wherein the path of the trajectory is measured satellite-supportedly on board a ballistically or quasi-ballistically fired projectile by increasing its aerodynamic drag coefficient so as to cause it to turn from an initial trajectory path into a steeper transitional trajectory towards the target.
2. Discussion of the Prior Art
A process of that kind is known from WO 98/01719. It is based on a procedure of using a satellite navigational apparatus on board the projectile to determine the trajectory which is currently being followed, and, from a comparison with a target-optimised trajectory, when a point on the trajectory which is derived from the comparison is reached, releasing aerodynamic braking devices for correction with the greatest possible degree of target accuracy of the subsequent trajectory. Problems arise in terms of practical implementation however by virtue of the fact that the numerous external influencing factors acting on a trajectory path still act on the trajectory even after the braking means are released and therefore the corrected trajectory does not then result in the operative mechanism in the projectile being delivered accurately on the target.
It is known from EP 0 138 942 B1 to locate a target for example by means of radar from the cannon and to determine in the fire control computer elevation and charge for a ballistic trajectory path which extends somewhat beyond the target, then to measure the launch speed of the projectile from the barrel and shortly thereafter by means of radar to ascertain the instantaneous position of the projectile relative to the cannon. Comparison of that instantaneous position with the reference position, on the basis of the calculated ballistic trajectory path, is used to determine the target layoff which is actually to be expected, the final step being to derive therefrom when aerodynamic braking effects should be activated at the projectile such as extending braking flaps or blowing off an aerodynamic projectile tip in order to suitably reduce the remaining trajectory on the basis of the new aerodynamic conditions and thereby to reduce the layoff from the target. This procedure also again only involves comparing a real to a predetermined ideal trajectory path in order to determine the attainment of a braking time so that once again the initialisation time for the braking means is error-ridden in dependence on external influences and then the interference effects which thereafter still act on the modified trajectory necessarily result in an additional target layoff.
Such a correction measure in respect of the braked transition from an initial trajectory path into a trajectory which is optimised after the apogee thereof is all the same substantially less expensive than the installation of a target sensor, control system and regulating loop for automatic, target-seeking final approach flight of a projectile. On the other hand, in consideration of the projectile speed being high in particular in the initial phase, the procedure for determining the real trajectory path from the measurement of initial instantaneous points on the trajectory is highly imprecise. The trajectory path which is actually flown however should be known to a very high degree of accuracy in order to be able to provide for optimum timing, after the apogee, of the braking manoeuvre for reducing the trajectory for the purposes of achieving a lower degree of scatter in the target area. Another problem in regard to a ground-supported process is also the reliability of a communication link for transmitting the braking triggering time or directly the braking command from the firing control computer to the projectile as, in view of the high speed of the projectile, the projectile can fly at any event in some sections of its trajectory in an ionised atmospheric shell which adversely affects a radio communication.
In consideration of those factors, the object of the present invention is to develop the process of the general kind set forth, which in itself is promising but which is still too inaccurate for the aspects of a practical situation, in such a way that it is possible to achieve substantially more precise target acquisition by way of a reduction in trajectory, as a result of an increase in the aerodynamic braking moment.
Accordingly the procedure according to the invention is based on the notion, as is known per se as such, of reducing the longitudinal scatter, which is very much greater in comparison with transverse scatter, of a ballistically or quasi-ballistically delivered projectile, in that the holding point is firstly laid behind the measured target position and then that trajectory is shortened. However, that laying effect is now only effected to such an extent that the transitional trajectory guides the projectile precisely on to the target after braking of the projectile having regard to a current error budget, on the theoretically shortest trajectory, wherein in accordance with the invention that given error budget is determined for as long as possible along the trajectory path to the braking moment from a comparison with the trajectory path which is theoretically predicted for given error parameters.
The projectile may be for example a drive-less projectile or missile which is fired from a mortar or from a howitzer, but also for example an artillery rocket with its rocket motor which acts to increase the range initially along a quasi-ballistic trajectory. The real transitional trajectory into which the projectile is then moved from its initial trajectory path by means of the aerodynamic braking effect lies between the flattest or shortest (minimum) and the highest or longest (maximum) trajectory of the current scatter fan or range and in principle can be converted by the braking action into the shortest trajectory, that is to say the trajectory which leads directly to the target.
Determining the current trajectory path does not involve having recourse to the procedure for determining the trajectory from the cannon, which is inevitably really inaccurate and technically unreliable due to interference effects. On the contrary, as is known per se, the initialisation point for the braking manoeuvre is autonomously determined on board the projectile, without therefore also being reliant for that purpose on a data link to a ground station. For that purpose the projectile is again equipped with a satellite receiving device for determining the actual initial trajectory path. As a deviation from the state of the art of the general kind set forth, the braking manoeuvre however is now not already triggered when a predetermined point on the trajectory is reached, but in accordance with the invention the initial trajectory path is compared to the theoretical launch curve over a period of time which is as long as possible, for as many trajectory points as possible. The build-up of the trajectory deviations which are ascertained therefrom, system-governed determining factors and preferably additionally measurements by sensor means for example on board the projectile and/or from the ground, such as in particular in accordance with DE 41 20 367 A1, are used as the basis for parametric determination of the current interference influences. These are in particular wind directions and strengths at different heights but also for example the error budget of the launch device (known transverse and heightwise aiming inaccuracies of the cannon) and influences of the intensity of the launch or firing charge, which varies depending on environmental considerations. With such knowledge, it is then possible by means of the usual external-ballistics approaches to pre-calculate really accurate information about the interference effects which even after release of the braking means still continue to act on the transitional trajectory which the projectile then follows, in order to compensate for those error influences to be expected as far as possible in advance by correction of the braking time. In order to obtain as much information as possible for the purposes of determining the current error budget, the braking time is as late as possible. Thus ultimately it is not defined in dependence on the launch of the projectile but in dependence on the remaining flight time to theoretical attainment of the target. It is therefore determined rearwardly in respect of time, so-to-speak in opposite relationship to the temporal motion along the trajectory.
In order to require as little flight time as possible for contacting the navigational satellites from the projectile, and in particular to cause the procedure for determining the real trajectory path to begin as soon as possible after projectile launch, the projectile is also given an item of information about the trajectory path which can be calculated for the instantaneous already known error budget, that is to say the currently ideal trajectory path, and about the satellite contacts which are to be expected therefrom. In that way it is possible very rapidly to access from on-board the projectile at least some of the navigational satellites which are above the horizon and rapidly obtain reliable information about the actual (real) trajectory path, that is to say also the deviation thereof from the trajectory which is predetermined by calculation, in order to infer therefrom the actual current error influences.
The greater the number of current trajectory points that can be measured on board the projectile by means of satellite navigation, the correspondingly more accurately is the trajectory path determined up to the time of initiation of the braking manoeuvre beyond the apogee, and the correspondingly more accurately is it therefore also possible to determine on board the layoff which is to be expected therefrom, from the conventionally measured moment which is communicated upon launch to the projectile. That makes it possible to suitably accurately predetermine the ideal initialisation point for initiation of the braking procedure, that is to say for entry into the transitional trajectory which is determined by the new aerodynamic conditions, from the real trajectory which has been predetermined as being too far, into the minimum, accurately targeted trajectory, in dependence on the remaining flight time into the target area. Because on the other hand that braking time which is as late as possible can be accurately determined, satellite tracking can be used for updating the knowledge about the real trajectory into the directly time proximity of the activation point for the braking manoeuvre, that is to say it can also be extended correspondingly long beyond the apogee, which results in a further improvement in determining the externally influenced real trajectory into the closest possible proximity to the target, and thus affords knowledge about the interference influences until close before the target. When then, on the real trajectory which is very accurately determined by continuous updating, for the currently prevailing error influences, the last possible initialisation point for entry into the braked transitional trajectory for the approach to the minimum trajectory is directly imminent, the structurally predetermined braking manoeuvre is triggered for example by extending braking elements or blowing off the aerodynamic projectile tip and therefore target acquisition is achieved with a high degree of reliability in the final approach flight phase, on the minimum trajectory or at any event on a trajectory which leads very close to the target.
In order to minimise the computation complication and expenditure for determining the optimum (latest possible) braking triggering point on board the projectile, desirably trajectory co-ordinates of an array of real trajectories which are to be expected, between the maximum and the minimum trajectories, and which are also displaced out of the pure trajectory parabola for example under wind influences or due to other interference influences, are stored in the form for example of look-up tables for example from the firing control computer in the processor on board the projectile; and in addition, as the triggering curve, the sequence of ideal, that is to say latest possible initialisation points over the remaining transit time of the respective trajectory of that array. For the current real trajectory within that array, which is then currently very accurately determined from satellite navigation, only the immediately imminent point of intersection of the currently flown, real trajectory with that triggering curve now still needs to be predicted, in order then to enable triggering of the braking effect for the transition into the accurately targeted minimum trajectory.